Utilizing Autoxidation of Solvents To Promote the Formation of Uranyl Peroxide Materials

Uranyl peroxide compounds are important to the nuclear fuel cycle and are typically formed from the direct addition of hydrogen peroxide or through photochemical pathways that utilize a sacrificial solvent. In this study, we explore the use of THF as a peroxide forming solvent that could be used to promote the crystallization of uranyl peroxide materials. Compound 1 (Li(H2O)2(UO2)(C7H3NO4)(C4H5NO4)·2H2O) was originally crystallized after aging for 10 days at room temperature and was characterized by X-ray diffraction, elemental analysis, Raman spectroscopy, and UV/vis spectroscopy. Exposure to ambient light for one month led to the complete transformation of 1 to compound 2 ((C5NH6)2[(UO2)2(μ-O2)(H2O)2(C7NO4H3)2]·4H2O). Oxidation of the THF molecule was proposed as the major mechanism for the formation of the secondary uranyl peroxide compound, which was accelerated in the presence of the UO22+ cation and light. Presence of THF hydrogen peroxide was confirmed by spectral (NMR and Raman) analysis of the mother liquor. Compound 3 ((C4N2H12)[(UO2)2(μ-O2)(H2O)2(C7NO4H3)2]·2.75H2O) formed in the presence of transition metal cations and additional experiments indicated that photoexcitation was not necessary to create free peroxide in this system. Autoxidation of the THF molecule is the most likely mechanism for peroxide formation and indicates that judicial choice of solvent can lead to the formation of new uranyl peroxide materials.